Coherent view of crystal chemistry and ab initio analyses of Pb(II) and Bi(III) Lone Pair in square planar coordination

The stereochemistry of 6s2 (E) lone pair of divalent Pb and trivalent Bi (PbII and BiIII designated by M*) in structurally related PbO, PbFX (X= Cl, Br, I), BiOX (X= F, Cl, Br, I) and Bi2NbO5F is rationalized. The lone pair LP presence determined by its sphere of influence E, equal to those of oxygen or fluorine anions, was settled by its center then giving M*-E directions and distances. Detailed description of structural features of both elements in the title compounds characterized by [PbEO]n and [BiEO]n layers allowed to show the evolution of M*-E distance versus the changes with the square pyramidal SP coordination polyhedra. All are different, in red PbO one finds {PbEO4E4} square antiprism, a {[Bi.E]O4X4Xapical} monocapped square antiprism in PbFX and BiOX and {BiEO4F4}square antiprism in Bi2NbO5F. To analyze the crystal chemistry results, the electronic structures of these compounds were calculated within density functional theory DFT. Real space analyses of electron localization illustrate a full volume development of the lone pair on PbII within {PbEO4E4} in PbOE, {PbEF4X4} in PbFXE and Bi(III) within {BiEO4X4} square antiprisms, contrary to Bi(III) within {[Bi.E]O4F4Fapical} monocapped square antiprism. Larger hardness (larger bulk modules B0) and band gap characterize BiOF versus PbO due to the presence of F which brings antibonding Bi-F interactions oppositely to mainly bonding Bi-O. In PbFX and BiOX series there is a systematic decrease of B0 with the increasing volume following the nature and size of X which is decreasingly electronegative and increasingly large. The electronic densities of states mirror these effects through the relative energy position and relative electronegativities of F/X and O/X leading to decrease the band gap.Comment: 20 text pages/ 10 multifigures/large review article, J. Progress Solid State Chemistry, under production MAY 201

Novel Carbon allotropes with mixed hybridizations: ene-C10, and ene-yne-C14. Crystal chemistry and first principles investigations

Based on C8, carbon 4C, with cfc topology, two hybrid carbon allotropes generated by inserting C(sp2) and C(sp1) carbon atoms into C8 diamond-like lattice were identified and labeled ene-C10 containing C(sp2) and ene-yne-C14 containing C(sp2 and sp1). The introduced double and triple chemical descriptions were illustrated from the projected charge densities. The crystal density and the cohesive energy were found to decrease due to the enhanced openness of the structures from inserted sp2/sp1 carbons, with a resulting decrease of the hardness along the series C8, C10, C12, and C14. The novel hybrid allotropes were found stable mechanically (elastic constants and their combinations) and dynamically (phonons band structures). The thermal properties from the temperature dependence of the heat capacity CV were found to increasingly depart from diamond-like C8 to higher values. From the electronic band structures, the inserted carbons were found to add up bands rigidly to diamond-like C8 while being characterized by metallic-like behavior for ene-C10 and ene-yne-C14.Comment: 18 pages, 32 references, 2 tables, 6 figure

Evolution from quartz (qtz) to diamond (dia) carbon allotropes: Crystal engineering and DFT investigations

Based on crystal engineering and density functional theory DFT calculations a transformational pathway is proposed from qtz (quartz-based) topology characterized by distorted tetrahedra to dia (diamond-like) regular tetrahedra topology. The protocol consists of carbon insertions into orthorhombic (space group P222, No. 16) within C5, C6, and C7, leading to ultimate C8 identified as diamond-like. The induced structural and physical changes are assessed with elastic properties pointing to ultra hardness, larger for qtz-C6 than dia-C8, whilst intermediate C7 is compressible due to its diamond-defective structure. The dynamic stability was shown from the phonons, and thermodynamic quantities as the specific heat CV was addressed in comparison with diamond experimental data. The electronic band structures reveal semi-conducting C6, metallic C7 characterized by diamond-defect structure, and insulating C8.Comment: 15 pages, 4 figures, 2 tables, 27 reference

C5 as simplest ultrahard allotrope with mixed sp2/sp3 carbon hybridizations from first principles

From crystal chemistry rationale and density functional DFT calculations, novel tetragonal carbon C5 is proposed as simplest ultrahard allotrope with mixed hybrid carbon hybridizations (sp2 - sp3). Novel pentacarbon is identified as cohesive and stable both dynamically and mechanically. Whereas charge density is localized about tetrahedral C-sp3, it is found delocalized around trigonal C-sp2 resulting in metallic behavior. The anisotropic structure characteristics caused by the aligned trigonal carbon along the tetragonal c-axis provide high Vickers hardness with a magnitude close to diamondComment: 15 pages, 26 refs, 2 Tables, 5 figs. arXiv admin note: text overlap with arXiv:2206.0591